Generally, an electro-magnetic speaker is the same as a dynamic speaker in respect of their operation principle, wherein magnetic field lines of alternating current generated from wire-winding coils interacts with magnetic field lines of direct current induced from a permanent magnet. However, differing from the dynamic speaker where the coils are directly driven, whereas the electro-magnetic speaker performs electro-acoustic conversion by fixing the coils and magnetizing an iron piece mounted to an end of the coils to vibrate the same.
The electro-magnetic speaker comprises a balanced armature speaker and a plate type speaker and KR 1596894 discloses the plate type speaker falling under the electro-magnetic speaker.
Referring to the electro-magnetic speaker of the above-mentioned KR patent, upper coils are arranged opposite to lower coils; one or more permanent magnets are located outside of the upper and lower coils; a vibration plate is disposed between the upper and lower coils; and peripheries of upper and lower surfaces of the vibration plate are supported by upper and lower damper members.
A process for driving the vibration plate of the above-described conventional electro-magnetic speaker will be described. First, provided that an upper face of the permanent magnet is the South pole and that a lower face of the permanent is the North Pole, the inner magnetic field direction of the upper coils and the inner magnetic field direction of the lower coils are opposed to each other in an inward direction, when electric currents are in a plus (+) period, and thus, the vibration plate moves upward because it becomes the North pole.
When the electric currents are zero, the vibration plate is neutralized and the vibration plate moves toward a neutral position by itself due to magnetic field balancing of the upper and lower permanent magnets and restoring force of the vibration plate.
When the electric currents are in a minus (−) period, the inner magnetic field direction of the upper coils and the inner magnetic field direction of the lower coils are opposed to each other in an outward direction, and thus, the vibration plate moves downward because it becomes the South pole.
In the above-described electro-magnetic speaker, the vibration plate is magnetized by electro-magnetic force of the upper and lower coils and it is directly driven in reaction to the magnetic force of the permanent force while coil weight is excluded. Accordingly, a fast response is obtained corresponding to electric sounds, so that it is possible to obtain very fine and detailed sound reproduction thereby.
Also, the vibration plate of the electro-magnetic speaker performs acoustic conversion, while it is actively driven by itself. Thereby, it is possible to minimize distortion rate of conversion path of the vibrator, so that it is advantageous to the more clear sound reproduction.
In the conventional electro-magnetic speaker described above, however, since the vibration plate is prepared as an integrated unitary vibration plate in a plate form, such as a circular plate or an elliptical plate, the extension of high tone-zone is restricted. If the vibration plate has the shape such as a dome and a cone, it is possible to cover the entire band pass. However, due to strong elasticity that is the property of amorphous alloy material, it is difficult to form and manufacture the vibration plate, and thus, cost increases.
Also, since the integrated unitary vibration plate in a plane shape is supported by upper and lower damper members and a gap guide is provided in their outside, a space for covering amplitude is needed and magnetic resistance increases. Accordingly, efficiency decreases and the thickness of the speaker increases.
Particularly, although the conventional electro-magnetic speakers have a better resolution, most of them show restrictive performance with regard to the extension in low tone sound and ultra high tone sound. Thus, an improvement is being required in order to meet the recent digital audio market, wherein musical elements of low and high tones are much included and are more refined and wherein the dynamic range extends. Accordingly, a combined-type multi-way speaker, which has a higher capability and a reproduction property of ultra broadband width, is required for the purpose of reproducing a superior quality sound.